The Age of the Dinosaurs

Artist's rendering of an asteroid striking Earth near the end of the age of dinosaurs. NASA.

Reptiles occupied the void left by the Great Dying and dominated animal life from 251 megayears (Ma) to 66 Ma (one megayear = one million years ago), an era known as the Mesozoic, or the Age of Dinosaurs. Angiosperms, true flowering plants, also first appeared and became prevalent. All of this biological activity undoubtedly benefited from a relatively stable, warm climate.

The Triassic period (251 Ma to 200 Ma), like the periods that preceded it, was dry and had large seasonal variations in temperature and precipitation. Low sea levels probably intensified seasonal variations. During the Jurassic period (200 Ma to 146 Ma), Earth’s crust expanded on the ocean floor, displacing water and causing sea levels to rise. Pangea began to rift into smaller continents, bringing more land area in contact with the ocean. With closer proximity to water, these continents had more stable temperatures and higher humidities. Climate during the Cretaceous period (146 Ma to 65 Ma) was uniform across the planet. The poles were ice free, and the deep oceans were at least 10°C warmer than today. Temperature gradients between the ocean’s surface and deeper water were small, diminishing currents and thereby the circulation of oxygen.

The Age of Dinosaurs ended about 65 Ma. In a relatively short time frame, all non-avian dinosaurs went extinct. But a closer examination of the fossil record reveals that a broad range of organisms, not just dinosaurs, went extinct within a time span of 500,000 years. For example, mammals also suffered, with marsupials and multituberculates (rodent-like mammals) experiencing heavy losses. More than 50% of all plant species vanished.

Geologic evidence suggests that these extinctions may have been precipitated by the impact of a massive asteroid near the Yucatan Peninsula, where a circular depression (180 kilometers in diameter) is discernible from space. An impact of the size that occurred in the Yucatan could blast enough dust and vapor into the atmosphere to block sunlight from reaching the surface of Earth for months, if not years. The planet would cool rapidly. Photosynthesis would be severely compromised, plant growth would stop, and animals would starve. Such an impact could also trigger volcanic eruptions and firestorms from incendiary fragments. As organisms died and decayed and as volcanoes and firestorms spewed gas, atmospheric CO2 concentrations would rapidly increase and thereby induce a temperature increase of several degrees once the dust settled.

Several issues remain controversial. [1], [2], [3] Some researchers date the impact on the Yucatan about 300,000 years before the K-T boundary (the geologic layer separating fossil evidence of the age of the dinosaurs from everything that came afterward). They also believe that the mass extinctions during this period transpired over many millennia, not just a few years. In their view, rock layers from around the world, which contain iridium at high concentrations and impact glass spherules and which date to the K-T boundary, indicate that a series of major asteroids struck Earth at around this time; others view these data as consistent with a single impact off the Yucatan Peninsula. To resolve this scientific controversy may take a reappearance of dinosaurs.